Direct modulation transmitter architecture, in which no intermediate frequency stage is utilized is finding increased popularity. A significant application is in cell phones and other wireless devices operating, for example, in the area of 1.9 GHZ. Direct modulation architecture is of interest because of its simplicity and lower number of external circuit components such as surface acoustic wave (SAW) filters and voltage control oscillators (VCOs). Also, since there are no intermediate frequency stages, the stages need not be driven. Consequently, the direct modulation transmitter provides for decreased power requirements. Efforts at integrating as many components as possible of a direct conversion transceiver onto a single chip have further led to increased popularity.
Since the number of intermediate frequency stages and components are minimized, direct modulation architecture has inherently provided difficulty in providing maximum isolation between the modulator output and an antenna. There is only one RF stage. It is highly desirable to provide for maximum gain control in the RF domain. Due to the close coupling of the load and modulator, pulling and injection lock of the local oscillator may be common occurrences.
One prior art approach to address these problems is by modulating on a sub-harmonic of the RF carrier frequency and performing signal control on the sub-harmonic frequency. Then, the sub-harmonic frequency is multiplied, and the signal control is also performed on signals at the desired frequency. This approach increases dynamic range.
However, this approach will cause large modulation distortions for non-constant signal envelope digital modulation and other systems using complex modulation. These forms of modulation include quadrature phase shift keying (QPSK) and some other forms of phase shift keying. Relevant cell phone technologies include CDMA (code division multiplex access), WCDMA (wideband CDMA), and WCDMA-2000 GSM (Global System for Multiple Communications) and EDGE (Enhanced Data Rates for GSM Evolution).